1. Field of the Invention
The present invention relates to devices making it possible to radially displace a rotated disk. Such devices are more particularly used in video disk readers in order to make it possible to read the track carrying the information by displacing said disk relative to a fixed reading head. The invention also relates to video disk readers equipped with such a device.
2. Description of the Prior Art
It is known to record on a disk-like support information representing, for example, a television signal. The information is carried by a spiral track, whose pitch is approximately a few microns. In order to read this track the disk is rotated at a relatively high speed, for example 1500 revolutions per minute or r.p.m. and a reading head equipped with an optical device is radially displaced in order to make it possible to read with a light beam the information carried by the track. This reading head has means, for example oscillating mirrors, making it possible to correct rapid radial and tangential fluctuations of the point to be read on the groove. These means function on the basis of reading signals in which are sampled adequate error signals.
The forward movement of the reading head is brought about by means of a forward movement mechanism. The movement of this mechanism is controlled by the radial error signal suitably filtered in such a way that the head follows the mean forward movement of the track, the radial fluctuations being corrected by the means referred to hereinbefore.
In certain cases it is desired to advance the head much more rapidly, for example when it is desired to look at a sequence recorded further on the disk. In this case the reading light beam no longer has to follow the track and jumps from one turn to the next. By counting these turn jumps it is possible to obtain information on the head location and stop the rapid forward movement at a given point corresponding to the information which it is desired to read.
Essentially two types of mechanism have been previously used for obtaining this radial movement of the head.
One of the mechanisms has an electromagnetic piston which moves under the action of a variable magnetic field produced in a coil. Such a device requires particularly heavy and expensive magnets and the power levels required for supplying the coil during the rapid displacement are high.
Another mechanism has a lead screw actuated by a motor. When the said screw rotates it drives a nut fixed to the reading head support making the head move forwards or backwards. To have a slow and a fast forward movement speed it is usually necessary to use two different motors and a clutch system making it possible to use one or other of said motors. This system is particularly complex, complicated and unreliable.